Plasma Process and Reactor for Treating Metallic Pieces

ABSTRACT

The plasma reactor defines a reaction chamber provided with a support for the metallic pieces and an anode-cathode system, and a heating means is mounted externally to said plasma reactor. The plasma process, for a cleaning operation, includes the steps of connecting the support to the grounded anode and the cathode to a negative potential of a power source; feeding an ionizable gaseous charge into the reaction chamber and heating the latter at vaporization temperatures of piece contaminants; applying an electrical discharge to the cathode; and providing the exhaustion of the gaseous charge and contaminants. A subsequent heat treatment includes the steps of: inverting the energization polarity of the anode-cathode system; feeding a new gaseous charge to the reaction chamber and maintaining it heated; applying an electrical discharge to the cathode; and exhausting the gaseous charge from the reaction chamber.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 12/737,125, filed on Jan. 18, 2011, the contents ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention refers to a plasma process and reactor for thetreatment of metallic pieces, particularly porous metallic piecesobtained by powder metallurgy, said treatment comprising a cleaningoperation with dissociation and removal of oil and other organic andinorganic contaminants existing on the surface or in the pores ofmetallic pieces, and generally also an operation of thermochemicallytreating the surface of said metallic pieces, which operations areeffected in a plasma reactive environment and, preferably, in theinterior of the same reactor.

BACKGROUND OF THE INVENTION

In most of the cases, the pieces produced by powder metallurgy need tobe calibrated after the sintering step due to the dimensional variationsthat occur during sintering. Lubricant oil is used in the calibration toreduce friction and wear of the machine tools, as well as to facilitateextraction of the pieces from the calibration matrix. Oil is likewiseused for storing sintered pieces and pieces produced by othermanufacturing technique. For example, refrigerant oil is used formachining high precision pieces.

Aiming at improving the properties of the finished pieces, such as wearresistance, corrosion resistance and fatigue resistance, there are oftenused surface thermochemical treatments, such as nitration, cementation,carbonitration, etc. In order to effect these thermochemical treatments,the presence of oil on the surface and in the pores of the pieces isprejudicial, especially when the thermochemical processing is effectedvia plasma. For example, during plasma nitration, the oil retained inthe pores and on the surface of the pieces produces instabilities in theelectrical discharge, contamination of the reactor, inadequate formationof the superficial layers formed (for example, nitrates) andcontamination with carbon of the material submitted to treatment bymeans of an inefficient cleaning. Thus, the oil must be completelyremoved before the thermochemical treatments of surface hardening.

Conventionally, a chemical cleaning is carried out in ultrasound withorganic solvents (for example hexane, petroleum ether or alcohol)further followed by a heat treatment in atmosphere containing hydrogenor oxygen in an industrial electric oven, aiming at eliminatingcompletely all the organic residues from the pieces. When communicatingresidual pores are present, which generally occurs in sintered steels,the cleaning is especially difficult, besides being pollutant due to thepollutant products used.

In some known treating methods, the operations of cleaning andthermochemically treating the surfaces are carried out in two separatesteps in distinct equipment, which requires a very long processing time,typically 20 hours, leading to low productivity and high cost.

With the purpose of obtaining a complete removal of the oil and otherorganic and inorganic contaminants from the surface or pores of themetallic pieces, and also simplifying and abbreviating a subsequentsurface thermochemical treatment operation of said pieces in the samethermal cycle, there has been proposed the process of cleaning andsurface treatment object of Brazilian patent application PI-0105593-3,of the same applicant, according to which the pieces to be cleaned arepositioned on a support provided inside the plasma reactor and connectedto an anode of the latter, the cathode of said reactor being connectedto a negative potential. The assembly defined by the support and piecesis surrounded by an ionized gas at low pressure and containing ions,neutral atoms and electrons, known as plasma and which is generated byan abnormal electrical discharge. The electrons provoke an electronicbombardment on the assembly defined by the support and pieces andconnected to the reactor anode.

The generation of gaseous plasma in the interior of the reactor allowsthe plasma reactive environment formed around the pieces to be used tocatalyze the reaction of dissociating the molecules of the oil and ofother possible contaminants existing in the pieces, allowing thevaporization of said contaminants and the complete elimination thereofthrough exhaustion, under vacuum, from the inside of the reactor. Theheat generated by the plasma, by the collision of fast ions and neutralatoms against the cathode, is usually sufficient to provide vaporizationof the molecularly dissociated oil, without requiring relevant changesin the plasma parameters more adequate to catalyze the reactions ofinterest in each cleaning operation.

However, in plasma reactors of certain dimensions (and in certainchemical reactions of molecular dissociation of the contaminants), itmay occur that different inner regions of the reactor remain at atemperature which is sufficiently low to allow condensation of thecontaminant vapors prior to the dissociation and progressive depositionthereof in these relatively cold inner regions of the plasma reactor,contaminating the system with carbon-based compounds that are harmful tothe subsequent surface treatments.

Moreover, in many surface thermochemical treatment operations occurringsubsequently to the cleaning operation and carried out inside the samereactor, the heat generated by the plasma is not enough to maintain theheating rate and the process temperature required to obtain the desiredsurface treatment.

Thus, even if the surface treatment operation has been executed, asdisclosed in said prior patent application, by reverting the electricalcircuit between the anode and the cathode, and also by surrounding thepieces with gaseous plasma of ions with high kinetic energy, and byapplying an electrical discharge to the cathode so as to provoke anionic bombardment on the pieces, there is a need to adjust the plasmaparameters, not as a function of the reactions of interest, but aimingto obtain temperature levels inside the plasma reactor that aresufficient and necessary for the desired surface treatment. In thiscase, the temperature variations required inside the reactor areobtained as a function of the electrical discharge parameters, but somesituations may exist in which the intensity of the electrical dischargerequired for the production of determined temperatures leads to theformation of electrical arcs in the reaction environment, causingsuperficial damages (marks on the pieces) and contamination by carbondeposits on the surfaces of the pieces, impairing the subsequentthermochemical treatments, besides the fact that the thermal gradientnegatively influences the formation and homogeneity of the formed layer.

The provision of a resistive heating in plasma reactors is known in theart.

In one of these known reactors, there is provided an external resistiveheating for removing the binders and possible contaminants from thepieces obtained by sintering. However, in this known construction, thepieces to be submitted to a treatment for removing binders andcontaminants are applied to the reactor cathode, leading to theformation of electric arcs and consequent contamination of the pieceswith carbon, which is harmful to the subsequent surface treatments.

In another known type of reactor disclosed in patent U.S. Pat. No.6,579,493, there is provided an inner resistive heating to obtain hightemperatures sufficient to remove the binders and certain contaminantsfrom the metallic pieces obtained by powder metallurgy and also toprovide sintering of the pieces. Nevertheless, the provision ofresistive heating in the interior of this type of reactor requires theuse of high cost materials, such as molybdenum and the provision of heatradiation reflecting elements between the inner resistance and thereactor wall, and also the provision of cooling in said outer wall. Thissolution is inadequate for cleaning the organic contaminants from thepieces under treatment, since it allows the volatile vapors of oils andother contaminants to be condensed and deposited on the cold regions ofthe heat radiation reflecting elements and on the reactor wall, beforethey are exhausted from the reaction environment, contaminating thelatter and the pieces contained therein with carbon-based compounds thatimpair the subsequent surface thermochemical treatments of the pieces.

From the above, there is a need for the provision of a solution whichallows obtaining, in the interior of the reactor, homogeneous and evenhigh temperatures as a function of the desired surface treatment, in away that is independent from the electrical discharge parameters thatare more adequate for catalyzing the reactions of interest in each case.

SUMMARY OF THE INVENTION

Thus, it is an object of the present invention to provide a plasmaprocess and reactor for treating metallic pieces by means of gaseousplasma and at temperatures that are generated and controlled in a waytotally independent from the plasma generation parameters.

It is another object of the present invention to provide a plasmaprocess and reactor, as mentioned above and which allow the plasmageneration parameters to be maintained in levels that are sufficient andadequate for catalyzing the reactions of interest, without leading tothe formation of electrical arcs in the reaction environment.

It is a further object of the present invention to provide a plasmaprocess and reactor, as mentioned above and which allow a cleaningoperation to be carried out through molecular dissociation throughgaseous plasma and through vaporization and exhaustion of thedissociated contaminants, by maintaining the interior of the plasmareactor at temperatures higher than those of condensation of saidcontaminants.

It is also another object of the present invention to provide a plasmaprocess and reactor, as mentioned above and which allow the operation ofthermochemically treating the surface of the metallic pieces to becarried out, also by gaseous plasma, in the same reactor in which thecleaning operation is effected and at temperatures that are obtained andcontrolled by preferably resistive heating.

These and other objects are attained through a plasma process fortreating metallic pieces in a plasma reactor defining a reaction chamberprovided with: a support; an anode-cathode system associated with anelectrical power source; an ionizable gaseous charge inlet; and agaseous charge exhaustion outlet connected to a vacuum system.

The plasma process for treating metallic pieces of the present inventioncomprises the following cleaning steps: a) connecting the support to thegrounded anode and the cathode to a negative potential of the electricpower source; b) positioning the metallic pieces on the support in theinterior of the reaction chamber; c) surrounding the support and themetallic pieces with an ionizable gaseous charge fed into the reactionchamber; d) heating the interior of the reaction chamber, from the outerside of the plasma reactor, at vaporization temperatures of contaminantsto be dissociated from the metallic pieces being treated in the interiorof the reaction chamber; e) applying an electrical discharge to thecathode, in order to provoke the formation of a gaseous plasma of ionswith high kinetic energy surrounding the metallic pieces and thesupport, and a bombardment of electrons in the metallic pieces formolecular dissociation of the contaminants; f) providing the exhaustionof the gaseous charge and of the contaminants maintained in gaseousstate, from the interior of the reaction chamber. In the cases in whichthe metallic pieces are submitted, after cleaning, to a thermochemicaltreatment, the plasma process for treating metallic pieces of thepresent invention comprises, after step “f” of the cleaning operation,the further steps of thermochemically treating the surface of themetallic pieces, in the same reactor, said steps comprising: g)inverting the energization polarity of the anode-cathode system, so thatthe support, with the metallic pieces, defines the cathode;h—surrounding the support and the metallic pieces with a new ionizablecharge fed into the reaction chamber; i) maintaining the interior of thereaction chamber heated, from the outer side of the plasma reactor, andconducting the temperature therein to the levels required in the desiredsurface thermochemical treatment; j) applying an electrical discharge tothe cathode, so as to provoke the formation of a gaseous plasma of ionssurrounding the metallic pieces and the support, as well as an ionicbombardment in the metallic pieces; and k) providing the exhaustion ofthe gaseous charge from the interior of the reaction chamber.

The present invention also presents a plasma reactor for treatingmetallic pieces and in which the process steps described above arecarried out, said reactor presenting a metallic casing defining,internally, a reaction chamber, as already described, and a heatingmeans mounted externally to the metallic casing, in order to heat thelatter and the interior of the reaction chamber. According to one aspectof the present invention, the heating means is formed by at least oneresistor in thermal contact with the metallic casing.

According to a particular aspect of the present invention, the supportcomprises multiple parallel and spaced apart ordering structures whichare electrically coupled to the same electrode of the anode-cathodesystem and intercalated by conducting elements coupled to the otherelectrode of the anode-cathode system, each of said ordering structurescarrying at least one metallic piece to be treated. In a preferredconstruction, the metallic casing portions, producing heat radiation tothe inside of the reaction chamber, are disposed according to adirection orthogonal to the mounting direction of the orderingstructures.

Even in the cases in which the surface treating operations to beeffected inside the reactor require temperatures that are not so high asthose required in sintering operations and which reach values of about1100 degrees C., the positioning of the heating means externally to themetallic casing of the reactor allows the latter to present a simplerand cleaner inner construction, avoiding the formation of intermediaryregions, between the heating means and said casing, which aresusceptible to the capture and subsequent deposition of contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, with reference to the attacheddrawings, given by way of example of embodiments of the invention and inwhich:

FIG. 1 represents, schematically, a plasma reactor constructed accordingto the present invention, illustrating some metallic pieces provided ona support mounted in the interior of said plasma reactor; and

FIG. 2 represents a simplified and rather schematic vertical sectionview of a plasma reactor constructed according to the present inventionand housing, inside the reaction chamber, a piece support comprising aplurality of horizontal ordering structures.

DESCRIPTION OF THE INVENTION

As mentioned above and as illustrated in the appended drawings, theinvention relates to a plasma process and reactor for treating metallicpieces 1, said process being carried out in a plasma reactor 10comprising a metallic casing 20, having an ionizable gaseous chargeinlet 21 e and a gaseous charge exhaustion outlet 22, said metalliccasing 20 defining, internally, a reaction chamber 23 inside which isusually positioned a support and an anode-cathode system 40, associatedwith an electric power source 50 external to the metallic casing 20. Areaction chamber 23 is coupled to a vacuum system 60 connected to theoutlet 22 of the metallic casing 20. The reaction chamber 23 ismaintained hermetic for plasma generation therewithin, the inlet 21 ishermetically coupled to an ionizable gas supply source (not illustrated)and the outlet 22 is hermetically coupled to the vacuum system 60.

The metallic casing 20 is preferably formed in refractory steel (as, forexample stainless steel AISI 310 or 309) and the support 30 inrefractory steel (as, for example stainless steel AISI 310 or 309), butother type of material can be used, depending on the adequate processtemperatures.

The metallic casing 20 presents a prismatic shape, for example, acylinder, having wall extensions 20 a which, in the cylindrical shape,comprises a surrounding side wall and an upper end wall 20 b.

The metallic casing 20 is inferiorly open so as to be removably andhermetically seated and locked on a base structure B to which areadequately mounted component parts operatively associated with thereactor and which will be described ahead.

The plasma reactor 10 of the present invention further comprises aheating means 70 mounted externally to the plasma reactor 10, that is,to its metallic casing 20, in order to heat the latter and the interiorof the reaction chamber 23, for example, producing heat radiation fromthe metallic casing 20 to the interior of the reaction chamber 23.

The plasma reactor 10 is also externally provided with an outer cover11, generally made of carbon steel coated with an adequate heatinsulating means (aluminade and silicade fibers, for example) presentingan adequate shape so as to surround laterally and superiorly theassembly defined by the metallic casing 20 and by the heating means 70,defining a heating chamber 13 around the metallic casing 20 and insidewhich is positioned the heating means 70.

The heating means 70 is generally formed by at least one resistor 71mounted in thermal contact with the metallic casing 20, inside theheating chamber 13 defined between the metallic casing 20 and the outercover 11. According to a way of carrying out the present invention, itis further provided a ventilation system 80 comprising at least one aircirculating means 81 generally positioned external to the outer cover 11and provided with at least one suction nozzle 81 a and at least onedischarge nozzle 81 b that are opened to the interior of the heatingchamber 13, said air circulating means 81 being able to produce acirculating airflow in at least part of the interior of the heatingchamber 13 and through the suction and discharge nozzles 81 a, 81 b. Incase the air circulating means 81 is mounted externally to the heatingchamber 13, the suction and discharge nozzles 81 a, 81 b take the formof tubular extensions opened to the interior of the heating chamber 13.

The ventilation system 80 can further comprise at least one airexchanging means 82, generally with a construction similar to that ofthe air circulating means 81 and also positioned externally to the outercover 11. The air exchanging means 82 is provided with at least onesuction nozzle 82 a and at least one discharge nozzle 82 b opened to theinterior of the heating chamber 13, said air exchanging means 82 beingalso connected to an air admission duct 83, generally opened to theatmosphere, and to an air exhaustion duct 84, generally opened to theatmosphere. The air exchanging means 82 can be constructed in anyadequate manner known in the art so as to provide a controlled supply ofatmospheric air to the interior of at least one respective region of theheating chamber 13, while it extracts and expels to the atmosphere,through the air exhaustion duct 84, a corresponding amount of heated airremoved from at least one respective inner region of the heating chamber13, allowing effecting a certain heating degree of the inner regions ofthe heating chamber 13.

The intensity of the air circulation or air exchange within the heatingchamber 13 can be achieved by different ways, such as, for example, byvarying the operational speed of a ventilating means, not illustrated,or by varying the positioning of the inner deflecting means, also notillustrated.

In the illustrated embodiment, there are provided multiple aircirculating means 81 which also perform the function of air exchangingmeans 82, said means being constructed with ventilating and deflectingmeans adequately controlled to allow performing any one of the functionsof air circulation and air exchange mentioned above.

In the construction illustrated in the attached drawings, the reactionchamber 23 is provided, superiorly, with an inlet 21 positioned in thevertical axis of the metallic casing 20 of the plasma reactor 10, inorder to homogenously distribute the ionizable gaseous charge from saidinlet 21. In this construction, the support 30 is formed by a pluralityof ordering structures 31 that are horizontally or substantiallyhorizontally disposed, thus defining piece support or mounting planesthat are orthogonal to the direction in which the gaseous charge is fedthrough the inlet 21, said ordering structures 31 having throughopenings to allow the gaseous charge to reach the pieces mounted in theordering structures 31 that are more distant from the inlet 21.

The ionizable gaseous charge is admitted to and exhausted from thereaction chamber 23 by command of control valves, not illustrated, whichare automatically driven, for example, commanded by a control unit orother specific control means (not illustrated), but said control valvescan be manually driven. While not illustrated either, the switching ofthe anode-cathode system can be carried out automatically or not, itbeing observed that these means for controlling the valves and theanode-cathode system are particular aspects that do not restrict theconcept presented herein.

In the embodiment illustrated and as mentioned above, the support 30comprises multiple parallel and spaced apart ordering structures 31electrically coupled to the same electrode 41 of the anode-cathodesystem 40 and which are interposed by conducting elements 42 coupled tothe other electrode 41 of the anode-cathode system 40, each of saidordering structures 31 carrying at least one metallic piece 1 to betreated.

The conducting elements 42 coupled to the other electrode are positionedin the interior of the reaction chamber 23, between the orderingstructures 31, by using any adequate support structure that can bedefined by structural columnar elements 32 of the support 30 itself thatcarries the ordering structures 31, it being only necessary to mountsaid conducting elements 42 electrically insulated in relation to thestructure of the support 30 provided with the respective orderingstructures 31.

According to the present invention and as exemplarily illustrated, theheating means 70 is arranged so as to heat adjacent wall extensions ofsaid metallic casing 20 and extending according to a direction, usuallythat coinciding with the direction of the height of the reaction chamber23 and which is orthogonal to the mounting planes of the orderingstructures 31. With this arrangement, the heat radiated from said wallextensions of the metallic casing 20 to the interior of the reactionchamber 23 follows a direction parallel to the mounting direction of theordering structures 31, making more efficient the distribution, amongthe ordering structures 31, of the heat radiated from said wallextensions of the metallic casing 20.

The anode-cathode system 40 has its electrodes 41 defined by the anodeand cathode of said energizing system. During the cleaning operation ofthe plasma process of the invention, the electrode 41, which defines theanode of the anode-cathode system 40, is coupled to the orderingstructures 31 of the support 30, in which the metallic pieces 1 arepositioned, said electrode 41 being grounded, whereas the otherelectrode 41 which defines the cathode of the anode-cathode system iselectrically coupled to the electric power source 50.

In the plasma processes in which after the cleaning operation there iseffected, in the same reactor, processes of thermochemical treatment,the electrode 41, which defined the anode of the anode-cathode system,is coupled to the power source 50, whereas the other electrode 41 isgrounded.

As described below, the present invention allows performing the cleaningand the thermochemical treatment (for example, nitration,carbonitration, cementation, oxynitration, oxycarbonitration andothers), in which both steps are aided by plasma, in the same equipmentand in the same thermal cycle and in which the anode-cathode system isconfined in the interior of the reaction chamber 23 in order to allowplasma generation and consequently utilize plasma reactive environmentto catalyze the reaction of dissociating the contaminant molecules foundin the metallic pieces 1, such as oil organic molecules in the cleaningoperation.

During the cleaning operation, the metallic casing 20 is heated jointlywith the gaseous charge, which is also submitted to a certain heatingdegree inside the reaction chamber 23 upon plasma generation. Theformation of the gaseous plasma of ions contributes to the heating ofthe interior of the reaction chamber 23 and to the vaporization of thecontaminants being dissociated, both in the cleaning operation and inthe thermochemical treating operation.

Besides contributing to the vaporization of contaminants in the interiorof the reaction chamber 23, the external heating of the latter allowsthe gaseous plasma formed therewithin to be obtained with less energyconsumption. The resistive heating, provided externally to the reactionchamber 23, avoids the existence of cold walls in the interior of thelatter, that is, in the environment in which the metallic pieces 1 aresubject to the plasma treating process. It is necessary to avoid theexistence of cold walls in the interior of the reaction chamber 23, forexample, in the initial phase of heating the metallic pieces 1, sincethe oil evaporated from the pieces being treated tends to deposit on thenot sufficiently heated inner regions of the reaction chamber 23.

Thus, the additional and generally resistive external heating avoids theexistence of walls or regions of the reaction chamber 23 presentingtemperatures lower than those of vaporization of the contaminants, thatis, of the vaporized oil, impeding the condensation and deposition ofcontaminants in these cooler regions of the reaction chamber 23, beforesaid contaminants are exhausted, by suction, through the vacuum system60, through the outlet 22 of the metallic casing 20.

For the cleaning operation, the metallic pieces 1 to be processed arepositioned on the ordering structures 31 of the support 30 mountedinside the reaction chamber 23, electrically insulated from its metalliccasing 20. During the cleaning operation, the support 30 defines theanode of the anode-cathode system 40, which anode is grounded, whereasthe conducting elements 42 are connected to an outlet of the electricpower source 50, in negative potential, acting as the cathode of theelectrical discharge. The interior of the reaction chamber 23 ismaintained at a sub-atmospheric pressure and with desired values for theformation of plasma in the cleaning operation, by using the vacuumsystem 60. A charge of ionizable gases is fed into the reaction chamber23, through inlet 21 of the metallic casing 20, before providing theelectrical discharge in the cathode of the anode-cathode system 40.

In a way of carrying out the present invention, the ionizable gaseouscharge, in the cleaning operation, comprises hydrogen, and it can alsocomprise a gaseous mixture containing hydrogen and at least one of thegases consisting of argon, nitrogen, or a mixture comprising oxygen andother gases, as for example, nitrogen. The selection of the processgases will depend on the nature of the substance to be eliminated fromthe metallic piece (for example, oil).

For example, the gaseous charge will comprise:

hydrogen, when the contaminants to be removed from the metallic piecespresent reactivity with hydrogen, or are based on hydrocarbon chainsthat are dissociated in gases based on carbon and hydrogen (methane(CH₄), for example);

oxygen, when the contaminants to be removed from the metallic piecespresent reactivity with oxygen, or are based on hydrocarbon chains thatare dissociated in gases based on carbon and oxygen (carbon dioxide(CO₂), for example);

mixtures of argon and hydrogen or oxygen, when a higher electron densityis desired for dissociating the contaminants to be removed from themetallic pieces; and

mixtures of nitrogen and argon or hydrogen or oxygen, when thecontaminants to be removed from the metallic pieces have theirdissociation facilitated by the mixture of said gases.

The dissociation of other contaminant bases is also possible with theprinciple of the present invention.

The main principle of the cleaning operation consists in dissociatingthe oil molecules by electron bombardment, resulting in lightermolecules or gaseous radicals which are eliminated from the reactionchamber 23 by exhausting the gaseous charge and contaminants from theinside thereof. In a way of carrying out the present invention, theexhaustion occurs under vacuum, via bombardment through the vacuumsystem 60, producing an efficient cleaning of the pieces, as well asmaintaining the interior of the reaction chamber 23 deprived of oildeposits and other contaminant products, mainly the organic ones, thecleaning operation being effected at low temperatures, in the range offrom about 30 degrees C. to 500 degrees C., depending on the nature ofthe contaminants to be eliminated.

In such embodiment, the support 30 and the metallic pieces 1 to betreated are surrounded by the plasma generated with the electricaldischarge and bombarded mainly by electrons generated in the plasma. Thesecond electrode 41, which in the cleaning step receives the electricaldischarge and actuates as the cathode, is bombarded mainly by ions andconsequently heated. As the heat produced in the cathode warms themetallic pieces 1, for the cleaning operation, the heating means 70,external to the reaction chamber 23, supplies the additional amount ofheat necessary to obtain the heating rate and temperature required toavoid condensation of the contaminants on the inner walls of thereaction chamber 23, said heating rate and temperature being programmedindependently of the plasma parameters. These plasma parameters areadjusted or programmed so as to catalyze the reaction of dissociatingthe molecules of the contaminant, such as for example, oil. As alreadymentioned, the formation of gaseous plasma can also contribute with partof the heating of the interior of the reaction chamber 23 required toavoid condensation of contaminants on the inner walls of the reactionchamber 23.

The use of a heating means 70, external to the reaction chamber 23,presents the advantage of allowing a homogeneous temperature to beobtained in the interior of the reaction chamber 23, as well as avoidingthe deposition of vapors and soot resulting from the plasma reaction inthe metallic pieces 1 inside the plasma reactor 10. Another advantage,resulting from the geometry used in the confined anode-cathode system 40is that the species generated in the plasma surround, completely, themetallic pieces 1, leading to an efficient removal of the contaminants,such as oil, from the metallic pieces 1.

The dissociation of the oil molecules produces lighter radicals andmolecules, which maintain the gaseous physical state at the workingtemperature and are pumped outwardly from the plasma reactor 10 throughthe vacuum system 60.

The contaminant vapor is discharged from the reaction chamber 23 jointlywith the other gases produced in the plasma operation, upon completionof the cleaning operation of the metallic pieces 1. Since there are noresidues inside the reaction chamber 23, because the oil and othercontaminants are completely eliminated by the molecular dissociationactivated by the active species generated in the plasma, the load ofmetallic pieces 1 can be treated inside the same plasma reactor 10, uponcompletion of the cleaning operation of said metallic pieces 1, byraising the temperature in the interior of the reaction chamber 23 tovalues compatible with those required in a determined thermochemicaltreatment.

In the solution of the present invention, the plasma reactor 20 furthercomprises a switching system 90, which allows inverting the polaritybetween the anode and the cathode of the anode-cathode system 40, sothat the metallic pieces 1, which during the cleaning operation withdissociation of oil and contaminants are necessarily connected to theanode, are connected to the cathode of the anode-cathode system 40 forthe thermochemical treatment by plasma.

With the present invention, the cleaning and thermochemical treatmentoperations carried out by plasma occur in the same plasma reactor, withno need of interrupting the heating.

After the cleaning operation, with the removal of contaminants,particularly oil, the thermochemical treatment operation is started inthe same plasma reactor 10, by introducing, through the inlet 21 of themetallic casing 20, a charge of ionizable gases into the interior of thereaction chamber 23, which can be similar to the one used in thecleaning operation or contain determined specific gases for the desiredthermochemical treatment, said new ionizable gaseous charge being fed tothe interior of the reaction chamber 23, so as to surround the support30 and the metallic pieces 1.

The ionizable gases of the thermochemical treatment operation are fedinto the interior of the reaction chamber 23, after exhausting the gasesand vapors of the cleaning operation therefrom.

The polarity between the cathode and the anode is then inverted, so thatthe metallic pieces 1 are connected to the cathode and, upon generationof electrical discharge in a gas mixture specifically defined for adetermined thermochemical treatment of the already cleaned metallicpieces, carrying out this treatment in the same plasma reactor 10 and inthe same thermal cycle.

It should be noted that the present thermochemical treatment process canpresent alteration in this sequence of steps of feeding a charge ofionizable gases and of inverting the polarity, without changing theresult obtained.

After the admission of a new gaseous charge and the inversion ofpolarity of the anode-cathode system 40, the thermochemical treatmentprocess further comprises the steps of: maintaining the interior of thereaction chamber 23 heated from the outer side of the plasma reactor 10and conducting the temperature therewithin to the levels required in thedesired surface thermochemical treatment; applying an electricaldischarge to the cathode, in order to provoke the formation of a gaseousplasma of ions surrounding the metallic pieces 1 and the support 30, andan ionic bombardment on the metallic pieces 1; and providing theexhaustion of the gaseous charge from the interior of the reactionchamber 23.

In the thermochemical treatment operation, the gaseous charge suppliedto the reaction chamber 23 comprises, for example: a gaseous mixture ofhydrogen and nitrogen, when the thermochemical treatment is nitration; agaseous mixture containing hydrogen, nitrogen and carbon, when thesurface thermochemical treatment is nitrocarburization orcarbonitration; a mixture containing hydrogen, argon and carbon, whenthe surface treatment is cementation; and a gaseous mixture containingoxygen, hydrogen, nitrogen, argon and carbon, when the surfacethermochemical treatment is oxynitration, oxynitrocarburization oroxycarbonitration. Other gases can be used, depending on the desiredthermochemical process.

For the thermochemical treatment operation, the support 30 is connectedto the negative potential of the electric power source 50, through theelectrode 41 which actuates as the cathode of the electrical discharge,whereas the electrode 41 which had the cathode function before isgrounded, actuating as the anode of the electrical discharge. After thisinversion of polarity, that is, with the metallic pieces 1 on thesupport 30 connected to the cathode of the electrical discharge, thedesired step of thermochemical treatment by plasma is carried out in thesame plasma reactor 10 and in the same thermal cycle. The gaseous chargeto be ionized in the reaction chamber 23 is submitted and maintained, ineach of the cleaning and thermochemical treatment operations, at asub-atmospheric pressure of the order of 1.33×10¹ Pascal (0.1 Torr) a1.33×10⁴ Pascal (100 Torr), which pressures are obtained by action ofthe vacuum system 60 comprising, for example, a vacuum pump. Thecleaning and heat treatment operations utilize DC electrical discharge,which can be delivered in an atmosphere under low pressure containing anionizable gas charge, as defined above, so as to produce electrons andreactive atomic hydrogen or other species, depending on the gasesutilized for plasma generation.

The process of the present invention, including the cleaning andthermochemical treatment operations by plasma, can be used for metallicpieces 1 produced by powder metallurgy or by other manufacturingprocesses (for example, machining, stamping, cold extrusion, andothers). The process of the present invention promotes a cleaning of themetallic pieces 1 in the plasma reactor 10 of the present invention in aperiod of time of about 3 hours, with the total time, including thecleaning operation and the thermochemical treatment (for example,nitration) being of about 6 hours. This processing time can be changed,for longer or shorter periods of time, as a function of the nature ofthe contaminant and of the thermochemical process.

The heating means 70, external to the reaction chamber 23, warms theinner walls of the metallic casing 20, avoiding deposition ofcontaminants, such as oil drops. It is possible to maintain the plasmastable and to dissociate the whole charge of contaminants from themetallic pieces by the plasma generation, as well as maintain thereaction chamber 23 deprived of oils and soot, allowing to givecontinuity to the heating and to subsequently carry out the process ofthermochemical treatment with the help of plasma in the same plasmareactor. By using the auxiliary resistive heating system and keeping thereaction chamber 23 at about 500 degrees C., there is no occurrence ofany type of deposition and soot formation, and the electrical dischargeis kept totally stable, allowing the complete removal of contaminants,such as oil, in approximately 3 hours, via molecular dissociation. Boththe final temperature and the cleaning time can be altered as a functionof the chemical nature of the contaminant.

After this cleaning step, in which the metallic pieces I were connectedto the anode, the polarity is inverted and the support 30 of themetallic pieces 1 is connected to the cathode.

After exhaustion of the gases used in the cleaning process, thetemperature in the interior of the reaction chamber is raised, forcarrying out the process of thermochemical treatment, to values betweenabout 350 degrees C. and about 900 degrees C.

Upon changing the charge of the gaseous mixture to, for example,hydrogen and nitrogen and heating the reaction chamber at temperaturesbetween 480 degrees C. and 590 degrees C., there is effected the plasmanitration, or other thermochemical treatment, in the same plasma reactor1 and in the same thermal cycle, resulting in the reduction of the totalprocessing time and energy consumption, thus reducing the productioncost. Other temperature ranges are possible (temperatures higher orlower than the range cited as an example) and that are defined as afunction of the type of thermochemical treatment to be performed.

The metallic pieces 1 were treated in the plasma reactor in anindustrial scale, in which the operations of plasma cleaning andthermochemical treatment, such as plasma nitration, were carried out inthe same thermal cycle. Said metallic pieces 1 were analyzed by opticaland electronic microscopy, as well as by X-ray diffraction analysis.Results show that the nitrated layer obtained is similar to thatobtained by a conventional process, that is, not effected in a singlethermal cycle and carrying out the cleaning operation by traditionalprocesses with organic solvents and removing heat using other equipment.However, it is important to point out that the total treatment time,when the treatment is made by this novel process using plasma to removeoil, is significantly shorter, resulting in higher productivity.Moreover, the process of removing oil by plasma does not use pollutantreactants such as hexane and others, which are traditionally used in thechemical cleaning process. A further advantage of the present process isrelated to the use of the confined cathode-anode system 40 which allowssmaller distances to be used among the pieces in the support, therebyallowing the provision of a greater amount of pieces in the same volumeof the reaction chamber 23 and/or the utilization of equipment withreduced dimensions for the same productivity, as compared to the otherknown prior art systems. Finally, the use of the same plasma reactor 10to perform the steps of cleaning (for example, oil removal) andthermochemical treatment (such as nitration), by using the confinedcathode-anode system with an external resistive heating, leads to asubstantial investment reduction.

While only one way of carrying out the present invention has beenillustrated herein, it should be understood that alterations can be madein the form and arrangement of the constitutive elements, withoutdeparting from the constructive concept defined in the claims thataccompany the present specification.

What is claimed is:
 1. Plasma reactor for treating metallic pieces andcomprising a metallic casing defining, internally, a reaction chamberprovided with: a support; an anode-cathode system associated with anelectric power source; an ionizable gaseous charge inlet; and a gaseouscharge exhaustion outlet connected to a vacuum system, characterized inthat it comprises a heating means mounted externally to the metalliccasing in order to heat the latter and the interior of the reactionchamber.
 2. Reactor, according to claim 1, characterized in that theheating means transfers heat to the interior of the reaction chamber byradiation from the metallic casing.
 3. Reactor, according to claim 2,characterized in that the heating means is formed by at least oneresistor in thermal contact with the metallic casing.
 4. Reactor,according to claim 3, characterized in that the support comprisesmultiple parallel and spaced apart ordering structures electricallycoupled to the same electrode of the anode-cathode system and which areintercalated by conducting elements coupled to the other electrode ofthe anode-cathode system, each of said ordering structures carrying atleast one metallic piece to be treated.
 5. Reactor, according to claim4, characterized in that the direction of the heat radiation producedfrom the metallic casing to the interior of the reaction chamber occursaccording to a direction parallel to that of the ordering structures ofthe support.